In regular double sideband amplitude modulation broadcast and communication as it has been practiced for nearly a century, a generated carrier frequency is modulated by varying the carrier's amplitude (voltage and current) in accordance with an audio frequency modulating signal.
The theory of amplitude modulation is well known. In a pure unmodulated sine wave carrier all of the power or energy is at the carrier frequency. Apart from circuit losses, the applied DC input power is radiated at that single carrier frequency. When amplitude modulation is applied, the resulting momentary departures from a pure sine waveform require additional power input, and this additional power becomes radiated as sidebands that are offset in frequency above and below the carrier frequency by an amount equal to the frequency of the modulating signal. A 1 mHz carrier that is amplitude modulated by a steady 1 kHz sine wave tone produces unmodulated sine wave sideband signals at 0.999 mHz and 1.001 mHz. To accommodate these sideband signals, channels of designated bandwidth, e.g. 10 kHz, are allocated in the broadcast band under FCC licence to AM broadcast stations, and each station is required to operate within regulations regarding adjacent channel interference.
Up to 100% modulation, any change of amplitude or frequency of the modulating tone will result in a corresponding change in the sidebands with regard to amplitude or frequency offset from the carrier, which remains constant.
At 100% modulation, with a single steady sine wave modulating tone, the transmitted envelope, as observed on an oscilloscope, is driven to twice its unmodulated level at the positive audio/envelope peaks and to zero at the negative audio peaks (envelope troughs). The total energy in the band/can be resolved mathematically into a pure carrier at the center frequency remaining at the original unmodulated level, and the two equal sidebands, each −6 dB relative to the carrier i.e half amplitude and thus one quarter power.
Thus modulating 100% increases the total power to 150%, i.e. 50% over the unmodulated carrier power: the total in-channel transmitted power is 1.5 times the unmodulated carrier power, while the total power in the sidebands is one half the unmodulated carrier power, and the power in each sideband is one quarter the unmodulated carrier power (−6 db). The foregoing conditions are confirmed when observed on a spectrum analyzer: the “constant amplitude” carrier appears at center frequency flanked by the upper and lower sidebands at half the amplitude of the carrier (−6 dB), and as the % modulation is varied, the amplitude of the sidebands vary accordingly while the carrier remains constant. Accordingly all of the desired communication information imparted by amplitude modulation resides in the sidebands and not in the constant carrier, which plays the role of a fixed “bias” and is utilized in the receiver as a frequency and amplitude reference for functions that are independent of audio modulation, such as signal strength and/or tuning indication, AGC (automatic gain control) and/or AFC (automatic frequency control).
If the modulation is increased beyond 100% in the typical two-quadrant modulator used in AM transmitters, the envelope peaks increase accordingly; however, at the troughs, the sine wave envelope shape is abruptly disrupted and its slope is interrupted to become a straight horizontal line, i.e. the “zero” line. This effect, commonly described as “pinch off”, produces at the “corners” at the start and finish points in time of the straight zero line, a spectrum of spurious emissions that typically ‘spatter’ beyond the band limits and potentially interfere with adjacent channels in violation of broadcast regulations. Therefore broadcast stations try to not exceed 100% negative modulation to avoid interference due to this “pinch-off” effect, while at the same time competitive forces dictate that the available carrier and modulating power be deployed as effectively and efficiently as possible to maximize listener coverage with a strong, clear and energetic-sounding signal. In recognition that the interference problem originates at “pinch-off” at the envelope troughs and not at the envelope peaks, regulations have been relaxed to allow some amount of “upward” modulation, e.g. to a level corresponding to 125% modulation as long as “pinch-off” and resultant out-of-band “splatter” interference are avoided.
Consequently, special audio processing using some form unbalanced or asymmetrical ALC (automatic level control), clamping, clipping, unbalanced or asymmetrical compression and/or expansion techniques have been utilized to accomplish some form of “upward modulation” while avoiding “pinch-off” interference, however such approaches typically introduce considerable non-linearity and distortion in the audio as a tradeoff for increased radiated sideband power.
Therefore there is an unfulfilled need for a modulation system that can increase the audible clarity and loudness of an AM transmitter by realizing the potential advantage of upward modulation beyond 100% within the transmitter's power capabilities while avoiding unwanted spurious interference from “pinch-off” effect and also avoiding distortion in the received audio due to non-linearity, particularly asymmetry, in the modulating process.